This invention relates to the operation of a thermostat system having a primary heat source, such as a heat pump and an auxiliary heat source. More specifically, this invention relates to the provision of heat at the desired comfort setting using such a system's auxiliary heat source.
Presently, it is known in the art to provide a heating system having two independent sources of heat, to a comfort zone whose temperature is being controlled by the system. The use of such a two heat source system is desirable for a number of reasons. Thus, for example, in U.S. Pat. No. 4,293,092 to Hatz et al. a heating system is shown which uses a heat pump for primary heat, as well as an additional heat source which is self-contained and separate from the heating network of the building. This separate system uses mechanical power which is then converted to heat power and is transmitted as additional heat to the primary heat carrier.
Japanese Patent No. 55-118543 (A) to Fushimi shows the use of two heaters, wherein the auxiliary heater is operated only when the heating load is large. In this case, both the primary and auxiliary heater function simultaneously.
Japanese Patent No. 62-84246 to Miyajima concerns an auxiliary heater attached to a heat pump in an air conditioning system, the heater being used to prevent rapid lowering of the room temperature.
Japanese Patent No. 55-3523 to Karato, discloses a multi-chamber air conditioner with an auxiliary heater which is energized only when the heat load required in the room is high, as is determined by a strong signal.
Thus, it appears that in the past two mentioned instances, the auxiliary heater operates to insure sufficient heat as needed when large heating loads are required.
In German Patent No. 197,806 to Lennarz, a central heating unit having a boiler and heat pump contains a regulator with ambient air temperature sensors and a change-over switch. The change-over switch is actuated by the ambient temperature sensor and functions to change the source of heat when a predetermined temperature is achieved. Thus, a single thermostat can be used for both heat sources.
In European Patent 0,007,497 to Eisberg, assigned to a common assignee of the instant invention, discloses an energy saving system using a heat pump and furnace. The heat pump, which is more energy efficient when ambient temperature is close to the desired comfort temperature, is used during periods of relatively high ambient temperature; when the ambient temperature falls to the point where it becomes more economical to use the furnace, the furnace takes over the heating function.
The disclosures above all relate to normal anticipated operations of a heating system having two heat sources. In particular, it is now common, for the conservation of energy to have a system wherein a primary heat source is activated at higher ambient temperatures and a secondary heat source at lower ambient temperatures. However, this energy efficient method of operation can cause a problem.
At times, in operation, it is possible for the primary heat source, such as a heat pump system, to either be locked out or disabled. As used hereinafter, a "lock out" of the primary heat source occurs when such heat source is inoperable due to a system malfunction, such as a failure of a safety function, e.g. pressure switch current protector, etc. The term "disabled" is used to describe an event which has the effect of overriding the primary heat source operation which is not related to a system malfunction, e.g. the operation of an optimizer that switches control to the secondary heat source based on ambient temperature level.
In either case, the comfort level in the home or other comfort zone, would then have to be maintained by the second stage or auxiliary heat that is provided. However, the system, in order to operate under normal conditions, is set so that this auxiliary heat is energized at a lower ambient temperature than the temperature at which the heat pump would have been energized, had it been operating. Thus, under normal conditions, the temperature will be maintained, using only the auxiliary heat source, at a lower level or setting than was intended by the person setting the thermostat control. The desired comfort level will thus not be maintained.